Project Summary. Over the past two decades, excess dietary intake and low physical activity have contributed to an increase in the prevalence of childhood obesity in the United States, affecting a third of children and disproportionately impacting minority and economically disadvantaged children. Given that obesity is a risk factor for health outcomes later in life, including cancer, early obesity prevention efforts are critical for population health. Out-of-school time (OST) programs are an important setting for addressing childhood obesity given that 10.2 million U.S. children are enrolled in afterschool. The Out-of-school Nutrition and Physical Activity (OSNAP) group-randomized trial demonstrated improvements in children?s vigorous physical activity, the healthfulness of foods and beverages served and consumed, and health- promoting program policies. Now that the OSNAP intervention has been rigorously tested and effectiveness has been established, there is a critical need to evaluate training models to disseminate this intervention for broad population reach and impact. Our long-term goal is to investigate the implementation and dissemination of evidence-based prevention interventions in OST settings. The overall objective of this proposal is to establish the effectiveness of two existing training models for scaling up the OSNAP intervention and understand the influence of context on effective implementation. In collaboration with the YMCA, we will conduct a 3-arm group randomized trial to compare two methods of delivering the learning collaborative with a control group. This application addresses the following specific aims: 1. Compare the effectiveness and implementation cost of two learning collaborative training models for the OSNAP intervention. We hypothesize that both training models will produce healthy changes in OST nutrition and physical activity practices as measured by a validated observational assessment, compared to the control group. Secondary outcomes of specific healthy practices, such as offerings of physical activity and water, and process outcomes, such as cost and acceptability, will vary by training model. 2. Use mixed methods to identify actionable factors within the implementation context that influence the effectiveness of the OSNAP intervention delivered by two learning collaborative training models. We hypothesize that programs with more supportive implementation contexts will more effectively implement OSNAP than programs with less supportive contexts. Qualitative data will help explain how aspects of the implementation context influence effective implementation of each training model in greater depth.